Defibrillators are implanted in patients susceptible to cardiac arrhythmias or fibrillation. Such devices provide cardioversion or defibrillation by delivering a high voltage shock to the patient's heart, typically about 500-750 V. High voltage capacitors are used in defibrillators to accumulate the high voltage charge following detection of a tachyarrhythmia. It is desirable to make implantable devices as small as possible, with slim, flat packages being desired for pectorally implanted defibrillators. Therefore, flat capacitors have been developed to avoid the disadvantages of traditional cylindrical aluminum electrolytic capacitors.
Such a flat capacitor is disclosed in U.S. Pat. No. 5,131,388 to Pless et al., which is incorporated herein by reference. Flat capacitors include a plurality of layers laminarly arranged in a stack. Each layer includes an anode and a cathode, with the anodes and cathodes being commonly connected to respective connectors. The layers may be cut in nearly any shape, to fit within a similarly shaped housing designed for a particular application. The capacitance of such a device is proportional to the number of layers, and to the area of each layer, providing significant design flexibility. However, current flat capacitors are not as manufacturable as would be desirable. Misalignment of layers can cause shorting between the edges of anodes and cathodes, and with the walls of a metallic housing. Precision assembly by hand is required, increasing manufacturing costs. The housing size must also be increased to provide tolerance for alignment errors, resulting in a bulkier device. The thin cathode layers must be efficiently joined together and coupled to one electrode of the capacitor.
The present invention overcomes the limitations of the prior art by providing a capacitor and a method for making the capacitor using ultrasonic welding to join the cathode layers together. The capacitor includes a housing defining a chamber with a step provided at an inner portion of the housing. A plurality of capacitor layers each including a cathode foil, a separator and an anode foil, each having alignment holes die cut therein to precisely fit with alignment elements housing, is positioned within the chamber, with the alignment holes mating with the alignment elements. The housing is electrically conductive for connection to the cathode foils. Each cathode foil includes a cathode tab which extends away from the foil. The cathode tabs are pressed together and ultrasonically welded to the housing step abutting the periphery of the interior of the housing.